1. Field of the Invention
The present invention generally relates to a driver, and more particularly, to a off-chip driver (OCD) providing output current.
2. Description of Related Art
In a circuit system, a semiconductor chip is able to produce different logical signals to control devices (loads) outside the chip. In general speaking, a semiconductor chip can includes an off-chip driver for delivering the above-mentioned logical signals to loads. An OCD usually includes P-channel field effect transistors (FETs) and N-channel field effect transistors as the major components thereof. Therese FETs are implemented by, for example, metal oxide semiconductor (MOS) transistors or complementary metal oxide semiconductor (CMOS) transistors.
The output signal of a conventional OCD can be sent to loads in driving current mode. Due to process variation however, the output current of a driver would be affected by the variation. Thus, a varied output voltage results in an up-drift or down-drift output current.
When an OCD is used in a dynamic random access memory (DRAM), the unstable driving current caused by a process variation may make a DRAM unqualified to meet the standard and specification worked out by Joint Electron Device Engineering Council (JEDEC). Moreover, the unstable driving current would affect the coupling compatibility of the DRAM with the loads (for example, coupling with a motherboard), which leads to a possible mistake with the data signal received by the loads.